1. Field of the Invention
The present invention relates to a novel non-aqueous electrolyte having superior battery cycle characteristics and battery characteristics such as electrical capacity, storage characteristic, and also relates to a lithium secondary battery using the same.
2. Description of the Related Art
In recent years, lithium secondary batteries have been widely used as the power sources for driving compact electronic devices, etc. Lithium secondary batteries are mainly composed of a cathode, a non-aqueous electrolyte and an anode. In particular, a lithium secondary battery having a lithium complex oxide such as LiCoO2 as a cathode and a carbonaceous material or lithium metal as an anode is suitably used. Further, as the electrolyte for a lithium secondary battery, a carbonate such as ethylene carbonate (EC) or propylene carbonate (PC) is suitably used.
However, a secondary battery having more superior battery cycle characteristic and battery characteristics such as electrical capacity has been desired.
A lithium secondary battery using a highly crystallized carbonaceous material such as natural graphite or artificial graphite as the anode sometimes suffer from breakdown of the electrolyte at the anode and an increase in the irreversible capacity or in some cases peeling of the carboneous material occur. The increase in the irreversible capacity or the peeling of the carbonaceous material occurs due to the decomposition of the solvent in the electrolyte during the charge thereof and is due to the electrochemical reduction of the solvent at the interface between the carbonaceous material and the electrolyte. In particular, PC having a low melting point and high dielectric constant has a high electroconductivity even at a low temperature. Nevertheless, when a graphite anode is used, there are problems that the PC cannot be used for the lithium secondary battery due to the decomposition thereof. Further, EC partially decomposes during the repeated charge and discharge thereof so that the battery performance is decreased. Therefore, the battery cycle characteristic and the battery characteristics such as electrical capacity are not necessarily satisfied.
The objects of the present invention are to solve the above-mentioned problems relating to an electrolyte for a lithium secondary battery and to provide a lithium secondary battery having a superior battery cycle characteristic and also superior battery characteristics such as the electrical capacity, charge storage characteristic under the charged condition and also to provide a lithium secondary battery using the same.
In accordance with the present invention, there is provided a non-aqueous electrolyte comprising (i) a non-aqueous solvent and (ii) an electrolyte salt dissolved therein and a disulfonate ester derivative having the formula (I): 
wherein R indicates a C1 to C6 alkyl group and X indicates a straight-chain alkylene group having a C2-C6 principal chain or a branched alkylene group having a C2-C6 principal chain with at least one side-chain composed of a C1-C4 alkyl group.
Further, in accordance with the present invention, there is provided a lithium secondary battery comprising (a) a cathode, b) an anode and (c) a non-aqueous electrolyte comprising (i) a non-aqueous solvent and (ii) an electrolyte salt dissolved therein and (iii) a disulfonate ester derivative having the formula (I); 
wherein R indicates a C1 to C6 alkyl group and X indicates a straight-chain alkylene group having a C2-C6 principal chain or a branched alkylene group having a C2-C6 principal chain with at least one side-chain composed of a C1-C4 alkyl group.
The disulfonate ester derivative having the formula (I) contained in the electrolyte has the role of being partially reduced and forming a passivation film at the surface of the anode carbonaceous material during the charging. Thus, it is believed that when an active, highly crystallized carbonaceous material such as natural graphite or artificial graphite is covered with a passivation film, the decomposition of the electrolyte is suppressed and the normal charging and discharging are repeated without impairing reversibility of the battery.
In the disulfonate ester derivative having the formula (I) contained in the electrolyte comprised of a non-aqueous solvent and an electrolyte salt dissolved therein, R is a C1 to C6 alkyl group, preferably those such as a methyl group, ethyl group, propyl group, butyl group, pentyl group, and hexyl group. The alkyl group may be a branched alkyl group such as an isopropyl group, isobutyl group, and isopentyl group.
Specific examples of a disulfonate ester derivative having the formula (I) are ethylene glycol dimethanesulfonate (i.e., R=methyl group, X=xe2x80x94(CH2)2xe2x80x94 in the formula (I)), 1,3-propanediol dimethanesulfonate (i.e, R=methyl group, X=xe2x80x94(CH2)3xe2x80x94), 1,4-butanediol dimethanesulfonate (i.e., R=methyl group, X=xe2x80x94(CH2)4xe2x80x94), 1,6-hexanediol dimethanesulfonate (i.e., R=methyl group, X=xe2x80x94(CH2)6xe2x80x94), 1,4-butanediol diethanesulfonate (i.e., R=ethyl group, X=xe2x80x94(CH2)4xe2x80x94), 1,4-butanediol dipropanesulfonate (i.e., R=propyl group, X=xe2x80x94(CH2)4xe2x80x94), 1,4-butanediol diisopropanesulfonate (i.e., R=isopropyl group, X=xe2x80x94(CH2)4xe2x80x94), propylene glycol dimethane sulfonate (i.e., R=methyl group, X=xe2x80x94CH(CH3)xe2x80x94CH2xe2x80x94), 1,2-butane diol dimethane sulfonate (i.e., R=methyl group, X=xe2x80x94CH(C2H5)xe2x80x94CH2xe2x80x94), 1,3-butane diol dimethane sulfonate (i.e., R=methyl group, X=xe2x80x94CH(CH3)xe2x80x94(CH2)2xe2x80x94), 2-ethyl-1,3-hexane diol dimethane sulfonate (i.e., R=methyl group, X=xe2x80x94CH2xe2x80x94CH(C2H5)xe2x80x94CH(C3H7)xe2x80x94), 3-methyl-1,5-pentane diol dimethane sulfonate (i.e., R=methyl group, X=xe2x80x94(CH2)2xe2x80x94CH(CH3)xe2x80x94(CH2)2xe2x80x94), 2-methyl-2,4-pentane diol dimethane sulfonate (i.e., R=methyl group, X=xe2x80x94CH(CH3)2xe2x80x94CH2xe2x80x94CH(CH3)xe2x80x94), 2,3-butane diol dimethane sulfonate (i.e., R=methyl group, X=xe2x80x94C(CH3)xe2x80x94CH(CH3)xe2x80x94)xe2x80x94, 3-methyl-1,2-butane diol methane sulfonate (i.e., R=methyl group, X=xe2x80x94CH(CH(CH3)2)xe2x80x94CH2xe2x80x94).
It should be noted that the two R""s in the formula (I) may be the same or may be different, but a disulfonate ester derivative having the same R""s is preferably used from the viewpoint of the easy synthesis.
If the content of the disulfonate ester derivative having the formula (I) is excessively large, the conductivity, etc., of the electrolyte changes and the battery performance sometimes is decreased. Further, if the content is excessively small, a sufficient passivation film is not formed and the desired battery performance is not obtained. Thus, a range of 0.01 to 50% by weight, in particular 0.1 to 20% by weight, based upon the weight of the electrolyte is preferred.
The non-aqueous solvent used in the present invention is preferably comprised of a high dielectric solvent and a low viscosity solvent.
Examples of the high dielectric solvent are cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC). These high dielectric solvents may be used alone or in any mixture thereof.
Examples of the low viscosity solvent are a linear carbonate such as dimethyl carbonate (DMC), methylethyl carbonate (MEC), and diethyl carbonate (DEC); an ether such as tetrahydrofuran, 2-methyl tetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane, and 1,2-dibutoxyethane; a lactone such as xcex3-butylolactone, a nitrile such as acetonitrile, an ester such as methyl propionate, and an amide such as dimethyl formamide. These low viscosity solvents may be used alone or in any mixture thereof.
The high dielectric solvent and low viscosity solvent may be freely selected and combined for use. It should be noted that the above high dielectric solvent and low viscosity solvent are used in a ratio of normally 1:9 to 4:1, preferably 1:4 to 7:3 by volume (i.e., high dielectric solvent:low viscosity solvent).
Examples of the electrolyte salt used in the present invention are LiPF6, LiBF4, LiClO4, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiC(SO2CF3)3, LiPF4(CF3)2, LiPF3(CF3)3, LiPF3(C2F5)3 etc. These salts may be used alone or may be used in any combination thereof. These salts are normally used in concentrations of 0.1 to 3M, preferably 0.5 to 1.5M.
The electrolyte of the present invention can be obtained by, for example, mixing the above-mentioned high dielectric solvent and low viscosity solvent, dissolving the electrolyte salt therein, and further dissolving the disulfonate ester derivative having the formula (I) therein.
The electrolyte of the present invention is used as a constituent of a lithium secondary battery. The constituents, other than the electrolyte, of the secondary battery are not particularly limited. Various constituents generally used in the prior art may be used.
For example, as the cathode material (i.e., cathode active material), a complex metal oxide of (i) at least one metal selected from the group consisting of cobalt, manganese, nickel, chrome, iron, and vanadium and (ii) lithium is used. Examples of such a complex metal oxide are LiCoO2, LiMn2O4, LiNiO2, etc.
The cathode can be prepared by mixing the cathode material with a conductive agent such as acetylene black or carbon black, a binder such as polytetrafluoroethylene (PTFE), and polyvinylidene fluoride (PVDF) to form a cathode paste, then coating this cathode paste on a collector such as aluminum foil or a stainless steel foil or lath, followed by drying, compression molding, and then heat treating at a temperature of at 50 to 250xc2x0 C. for about 2 hours in vacuo.
As the anode active material, lithium metal, lithium alloy and carbonaceous materials having a graphite-type crystalline structure capable of intercalation and deintercalation lithium such as thermally decomposed carbons, cokes, graphites (e.g., natural graphite, artificial graphite), combustion organic polymer substances, carbon fibers as well as composite tin oxides are used. In particular, carbonaceous materials having a graphite-type structure and having a lattice spacing (d002) of 0.335 to 0.340 nm is preferably used. The powdery materials such as the carbonaceous material are mixed with a binder such as ethylene propylene diene terpolymer (EPDM), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF) for use as an anode paste.
The configuration of the lithium secondary battery is not particularly limited. A coin battery having a cathode, an anode, a single layer or multiple layer separator and further a cylindrical battery, prismatic battery, etc. having a cathode, anode, and roll-shaped separator may be mentioned as examples. It should be noted that, as the separator, a known polyolefin microporous film, woven fabric, nonwoven fabric, etc. can be used.